This invention relates in general to line isolation and interference shielding systems for shielded conductors and in particular, to such line isolation systems including means for preventing dangerous voltage buildups across the line isolator.
It is well known to provide a line isolator and interference shield for a 75 ohm coaxial cable used in cable connected television systems. In such systems, the cable sheath is a metallic braid that serves as the return wire for the two conductor system. The cable system operator establishes a good "earth" ground connection to preclude the cable braid or sheath from "floating", which could give rise to potentially dangerous voltages. The operator also wants to isolate the cable ground connection from the low frequency power line ground at the user site. The isolator supplies both needs. Since splitting the ground connection provides an opportunity for high frequency electromagnetic energy to interfere with the television signals being carried by the cable, as is fully described in U.S. Pat. No. 4,399,419 issued to Pierre Dobrovolny and assigned to Zenith Radio Corporation, the line isolator includes means allowing passage of the high frequency television signals while absorbing external electromagnetic energy.
The interruption in the line isolator precludes the cable sheath from acting as a low frequency power return should, for some reason, the power line ground at the user site become faulty. Normally, the site ground is provided by a ground rod or water pipe. Occasionally the ground connection develops problems which manifest themselves in the resistance of the connection departing from zero (or from a very low value near zero). It is not uncommon for the low frequency power ground return circuit at the site to be completed through the cable sheath of the well-grounded cable system, with consequent overload and damage to the cable. With an isolator constructed in accordance with the above patent, that is not possible since there is a discontinuity between the user site ground connection and the grounded sheath of the cable.
It is well known that antenna systems are prone to static discharge voltage buildups as well as to energy buildups caused by lightning strokes and the like. The same is true for cable-connected systems where the cables are run above ground. In the event of a lightning strike, for example, current must be channelled to ground as quickly as possible. The line isolator of the above patent includes ceramic capacitor and inductor elements that provide a fairly substantial leakage path thereacross for preventing excessive voltage buildup across the isolator. However, for very brief time periods, there can be a fairly significant voltage buildup across the isolator before the leakage across the ceramic elements has had an opportunity to dissipate the energy to ground.
The invention precludes even "short term" buildup of excessive potential across the isolator, even in the presence of high potential energy, by providing a very low resistance path to ground when the potential across the isolator reaches a predetermined magnitude.